The preparation of amines by the reductive amination of aldehydes using nickel-based catalysts is well known as shown, for example, by Freifelder, Morris, Practical catalytic Hydrogenation, John Wiley & Sons, Inc. 1971, pages 333-345, and has been utilized to manufacture a variety of amines such as 2,2-dimethyl-3-aminopropanol from hydroxypivaldehyde. Japanese Patent Publication JP 91-10848 A discloses the reductive amination of FTHF with aqueous ammonia using nickel, palladium, and platinum catalysts. Although this patent document claims better selectivity for the aqueous system, the best selectivity achieved by the use of palladium or platinum catalysts was 77.4% with 5% palladium on carbon at 30.degree. C. a higher selectivity of 99.5% was reported for Raney nickel at 60.degree. C. All of the examples were batch autoclave experiments and no problems with product isolation or catalyst instability resulting from the use of Raney nickel were mentioned.
U.S. Pat. Nos. 4,963,672 and 5,068,398 describe the reductive amination of 5-formylvalerates in liquid ammonia using a supported ruthenium catalyst. These patents teach that the disclosed processes are carried out at relatively high total pressures of 40 to 100 bar (580 to 1450 pounds per square inch-psi). The examples contained in the patents employed total pressures of 98 to 99 bar. U.S. Pat. No. 5,068,398 discloses a comparative example that demonstrates leaching of nickel from a supported nickel catalyst. U.S. Pat. No. 5,166,443 describes a two-step process wherein certain 2,2-disubstituted-4-cyanobutanal compounds are converted to their corresponding diamines. The first step involves treating the cyanobutanal with ammonia in the presence of an acidic heterogeneous catalyst. The resulting material then is fed to a hydrogenation zone which preferably uses a ruthenium catalyst. Again, the pressures are relatively high at 50 to 500 bar (725-7250 psi), preferably 100 to 350 bar and most preferably from 150 to 300 bar. In the examples of U.S. Pat. No. 5,166,443, the hydrogenation zone is operated at a pressure of 200 or 250 bar.
Initial attempts to prepare 3-(aminomethyl)tetrahydrofuran (AMTHF) by contacting an aqueous solution of 3-formyltetrahydrofuran (FTHF) with ammonia and hydrogen in the presence of conventional Raney nickel or supported nickel catalysts did not give satisfactory results due to significant leaching of nickel into the product. In addition to losing catalyst, product isolation and purification becomes more difficult due to precipitation of nickel salts upon removal of the excess ammonia and water. During removal of the ammonia and water by distillation, pale green salts precipitate and must be removed by distillation, filtration, or both.
The FTHF from which AMTHF may be prepared preferably is used in the form of an aqueous solution. As is disclosed by W. A. Beavers in U.S. patent application Ser. No. 944,653 filed Oct. 6, 1997, FTHF may be prepared by hydroformylation of 2,5-dihydrofuran in the presence of a rhodium-phosphine catalyst and an organic, water-immiscible, hydroformylation solvent. Recovery of FTHF by distillation in the presence of the hydroformylation solvent results in the formation of hemiacetal oligomers and an overall significant yield loss. Thus, the FTHF product preferably is recovered by first extracting the hydroformylation product solution with water to produce an aqueous solution of FTHF. The use of this aqueous solution of FTHF in chemical processes wherein the FTHF is converted to other compounds such as AMTHF is a particularly efficient means for producing such other compounds.